1. Field of the Invention
The present invention relates to a method and apparatus for displaying radar information including a cursor or template for use with this displayed radar information. The invention is useful with Frequency Modulated Continuous Wave radar systems employing waveform agility, i.e., systems employing variable radar carrier frequencies and waveform repetition rates. One application for the invention is radar systems using Over-the-Horizon-Radars.
2. Description of the Related Art
Frequency Modulated Continuous Wave radar systems make use of the property that signals of slightly different frequencies give rise to beat frequencies. Radar return signals reflected from a moving target will be shifted in frequency due to the velocity of the target. This frequency shift is due to the phenomena known as the Doppler effect. Doppler shifted radar return signals give rise to beat frequencies. These can be detected and converted into a value of the velocity of the radar target. However, due to the nature of the generation of the beat frequencies, the beat frequency does not provide information as to whether the target is coming toward the radar or moving away. Previous radar systems requires measurements to be made of radar returns of successive radar dwells. The present invention provides a method of determining the velocity of a target and whether a target is closing or retreating from the radar with minimal computation of radar returns of successive radar dwells.
The present invention is useful with radars employing waveform agility. Such a radar will employ variable waveform repetition frequency and variable carrier frequency. These waveform parameters will be constant for each radar dwell but will vary intentionally from dwell to dwell. Waveform agility is desirable for many reasons including:
a) blind-speed unmasking to reveal targets masked by radar clutter return signal, PA1 b) countering radar jamming measures, and PA1 c) avoidance of other users of the frequency spectrum. PA1 c=the velocity of light; PA1 WRF=waveform repetition frequency; and PA1 f=carrier frequency. PA1 V.sub.amb =the ambiguous velocity; PA1 P=the region of velocity values for each velocity bin; PA1 V.sub.max =the maximum velocity displayed; and PA1 the function {arg} is the smallest integer greater than or equal to arg. PA1 V.sub.amb =the ambiguous velocity; PA1 P=the region of velocity values for each velocity bin; PA1 V.sub.max =the maximum velocity displayed; and PA1 the function {arg} is the smallest integer greater than or equal to arg.
After signal processing, the radar returns are displayed on a visual display as video data. Employing waveform agility affects the video data which is displayed on a visual display. It has been known to display the video data in terms of range, azimuth and Doppler space. A Doppler frequency within the Doppler space for fixed waveform repetition frequency and fixed carrier frequency can be considered as a velocity of a detected target. The Doppler frequency is effectively the velocity of a target and corresponds to the shift in frequency of the return signal from the transmitted signal. The term "Doppler frequency" will be used herein to indicate Doppler space data displayed in terms of velocity, or Doppler space data that can be displayed in terms of velocity. In the case of OTHR the video data is known to be displayed in terms to Azimuth, with nested range versus Doppler frequency.
To facilitate operator confirmation of tracks, a series of successive radar scans are presented in a cyclically animating sequence. This also has the advantage of combating target fading by allowing visual integration. For fixed waveform repetition frequency and carrier frequency the Doppler frequency axis of a display can be interpreted as velocity with respect to the radar. Targets with a constant radial velocity with respect to the radar will maintain a fixed position in the Doppler space.
The stationarity of the targets in Doppler space is effected by factors other than target movement. If the waveform parameters change, then this stationarity is lost. In practice, the stationarity will only exist for a single dwell as waveform parameters can change from dwell to dwell. Consequently, a target will appear to have a varying Doppler frequency and the target display position will jitter on the visual display. The variations in the radar parameters lead to an ambiguous velocity range V.sub.amb. This can be written as; ##EQU1## Where: Vamb=ambiguous velocity;
As can be seen from the above equation, changes in the ratio of waveform repetition frequency to carrier frequency result in changes in the ambiguous velocity range. If the carrier frequency is varied, then the Doppler frequency varied. Previously, if the waveform repetition frequency varies and the Doppler frequency remained constant, then the target position on a display would vary. This is because the ambiguous velocity will vary and previously the display screen window size was constant for a specific ambiguous velocity.
As mentioned above, Over the Horizon Radar systems make use of the propagation characteristics of ionospheric propagation of radio waves. So the received radar returns are subject to the known characteristics of ionospheric propagation. As such, it is not uncommon for a radar return to be subject to fading and other atmospheric effects. The above is compounded with radar returns travelling via E layer, F layer or mix-mode paths. As a result, it can be very difficult to determine whether a radar return is a target or a spurious return.